This invention relates generally to computed tomography (CT) image processing, and more particularly, to reducing cone beam artifacts in reconstructed images.
Cardiac computed tomography (CT) scans typically use low pitch acquisitions that are retrospectively gated using cardiac EKG signals. Typical multi-slice CT EKG-gated cardiac reconstruction uses half-scan data corresponding to a portion of the cardiac cycle to achieve high temporal resolution needed to reduce cardiac motion. In half-scan mode, projections over the projection angle of π+2γm are used instead of 2π, where γm is the fan angle of the detector.
To get the necessary coverage of the heart, data is acquired for several heartbeats, with each heart cycle providing data for multiple slices. Data may be acquired helically, although from a reconstruction point of view, the data is similar to data acquired in step-and-shoot mode in that one data stream provides a range of coverage along the Z-axis. For step-and-shoot acquisition mode, it is known that the completely sampled region forms less than a cylindrical disc, or forms an incomplete cylindrical disc, with the disc's height equal to the detector iso-center coverage. Near the source (x-ray tube), the cone beam geometry reduces the coverage of each projection, resulting in a volume which is narrower than the desired volume. Therefore, during reconstruction, a portion of the cylinder that is closer to the source is extrapolated, such as by using the last row of known data in areas lacking data. Extrapolation of data can increase the artifact level significantly for the half-scan case, resulting in distorted structures and undesired shading.
Typically, the helical pitches for cardiac scans are set relatively low to avoid the use of the extreme edge of the detectors in the Z, or patient, direction. The pitch is not set too low to avoid exposing patients to unnecessary radiation. Depending on the application, pitches that allow more than 80% detector usage are selected to balance image quality, coverage and dose.
For a given Z-location during a cardiac helical scan there is a heart cycle whose corresponding detector position is closest to that particular Z-location. Due to the low pitch used during scanning, however, there may be a second heart cycle whose detector positioning covers substantially the same Z-location, though the center of the detector during the second heart cycle may be further away from the imaging location. As image quality degrades with the increase of distance from the center of the detector, the first heart cycle may be used to reconstruct the image using standard single sector reconstruction. Therefore, for Z-locations imaged in more than one heart cycle, a first image of a first Z-location may be preferable from a first heart cycle and a second image of a second Z-location may be preferable from a second heart cycle. With the current single sector reconstruction algorithm, some cone beam artifacts will show up for slices that are closer to the edge of the detector for a specific cardiac cycle.
Therefore, a need exists for reconstructing images while reducing or eliminating cone beam artifacts. Certain embodiments of the present invention are intended to meet these needs and other objectives that will become apparent from the description and drawings set forth below.